If the child is still under 21 to the time the visa application is adjudicated, you do not need Sec.3 of the cspa.

On the other hand, it apply when the petition visa number became available, the child is 21 or older. Here, you have to determine the reciept date and the approval date of the petition (1-130), add the number of days between it and subtract it from the age of the child at the time visa number became available.

Example: if the number of days between reciept date and approval date is 6 months and the child age is 21 years minus 6 months equals 20 years and 6 months. Under the cspa, the child is considered below 21 and will qualify to 2A but he or she has to seek LPR status within one year from the date of visa avaialability.